docs: def _get_site_type_dummy_variables(self, sites):
          """
          Get site type dummy variables, ``Ss`` (for soft and stiff soil sites)
          and ``Sa`` (for rock sites).
          """
          Ss = np.zeros((len(sites.vs30),))
          Sa = np.zeros((len(sites.vs30),))
          # Soft soil; Vs30 < 360 m/s. Page 199.
          idxSs = (sites.vs30 < 360.0)
          # Stiff soil Class A; 360 m/s <= Vs30 <= 750 m/s. Page 199.
          idxSa = (sites.vs30 >= 360.0) & (sites.vs30 <= 750.0)
          Ss[idxSs] = 1
          Sa[idxSa] = 1
          return Ss, Sa